Add differenceHyab(), fixes #126

docs/api.md

@@ -673,14 +673,20 @@ Computes the [CIE94][cie94] ΔE\*<sub>94</sub> color difference between the colo
 
 Computes the [CIEDE2000][ciede2000] ΔE\*<sub>00</sub> color difference between the colors _a_ and _b_ as implemented by [G. Sharma](http://www2.ece.rochester.edu/~gsharma/ciede2000/).
 
-Returns a [CIEDE2000](https://en.wikipedia.org/wiki/Color_difference#CIEDE2000) Delta E\* function.
-
 <a name="differenceCmc" href="#differenceCmc">#</a> culori.**differenceCmc**() &middot; [Source](https://github.com/evercoder/culori/blob/master/src/difference.js)
 
 Computes the [CMC l:c (1984)][cmc] ΔE\*<sub>CMC</sub> color difference between the colors _a_ and _b_.
 
 ΔE\*<sub>CMC</sub> is not considered a metric since it's not symmetrical, that is the distance from _a_ to _b_ is not always equal to the distance from _b_ to _a_. Therefore it cannot be reliably used with [`culori.nearest()`](#nearest).
 
+<a name="differenceHyab" href="#differenceHyab">#</a> culori.**differenceHyab**() &middot; [Source](https://github.com/evercoder/culori/blob/master/src/difference.js)
+
+Computes the HyAB color difference between the colors _a_ and _b_, as proposed in:
+
+> Abasi S, Amani Tehran M, Fairchild MD. _Distance metrics for very large color differences._ Color Res Appl. 2019; 1–16. https://doi.org/10.1002/col.22451 ([PDF](http://markfairchild.org/PDFs/PAP40.pdf))
+
+The HyAB formula combines the Euclidean and [city block](https://en.wikipedia.org/wiki/Taxicab_geometry) distance and has been experimentally shown to work better for large color differences than CIEDE2000, while still holding up well for smaller color differences, making it a _"good candidate formula for image processing and computer vision applications"_.
+
 ### Other difference formulas
 
 <a name="differenceDin99o" href="#differenceDin99o">#</a> culori.**differenceDin99o**() &middot; [Source](https://github.com/evercoder/culori/blob/master/src/difference.js)

src/difference.js

@@ -253,6 +253,31 @@ const differenceCmc = (l = 1, c = 1) => {
 	};
 };
 
+/*
+
+	HyAB color difference formula, introduced in:
+
+		Abasi S, Amani Tehran M, Fairchild MD. 
+		"Distance metrics for very large color differences."
+		Color Res Appl. 2019; 1–16. 
+		https://doi.org/10.1002/col.22451
+
+	PDF available at:
+	
+		http://markfairchild.org/PDFs/PAP40.pdf
+ */
+const differenceHyab = () => {
+	let lab = converter('lab65');
+	return (std, smp) => {
+		let LabStd = lab(std);
+		let LabSmp = lab(smp);
+		let dL = LabStd.l - LabSmp.l;
+		let dA = LabStd.a - LabSmp.a;
+		let dB = LabStd.b - LabSmp.b;
+		return Math.abs(dL) + Math.sqrt(dA * dA + dB * dB);
+	};
+};
+
 const differenceDin99o = () => differenceEuclidean('dlab');
 
 /*
@@ -278,6 +303,7 @@ export {
 	differenceCie94,
 	differenceCiede2000,
 	differenceCmc,
+	differenceHyab,
 	differenceDin99o,
 	differenceKotsarenkoRamos
 };

src/index.js

@@ -163,6 +163,7 @@ export {
 	differenceCie94,
 	differenceCiede2000,
 	differenceCmc,
+	differenceHyab,
 	differenceDin99o,
 	differenceKotsarenkoRamos,
 	differenceHueSaturation,

test/difference.test.js

@@ -5,6 +5,7 @@ import {
 	differenceCie94,
 	differenceCiede2000,
 	differenceCmc,
+	differenceHyab,
 	differenceKotsarenkoRamos,
 	rgb,
 	lab65,
@@ -181,3 +182,8 @@ tape('difference in LCh space', t => {
 	t.equal(differenceEuclidean('lch')('red', 'green'), 130.3764124621912);
 	t.end();
 });
+
+tape('differenceHyab', t => {
+	t.equal(differenceHyab()('red', 'green'), 139.93576718451553);
+	t.end();
+});